Lore

McDonnell is located in a secret room past a courtyard filled with giants, near where Pontiff Sulyvahn can be fought. Interacting with the Archdeacon allows you to join the Aldrich Faithful Covenant. He appears dead, but if you give him 30 Human Dregs, you can recover the Archdeacon's Great Staff from him.

At one time, McDonnell led the deacons of the Church of the Deep. Also a sorcerer, he found pleasure in the the languishing souls of the cathedral; for him, they were the foundation of this world.

McDonnell, like other Archdeacons, wielded a great staff, which he used to channel faith for casting sorceries. This was considered a sin though, as he used the clerical symbol of authority as a catalyst for sorceries.

Theories

Title of theory.

Sources

Cleric's Candlestick
Candle stick used as both sword and catalyst. Used for worship by the Deacons of the Deep.
The deacons, under the guidance of Archdeacon McDonnell, became both Clergymen and sorcerers.

Human Dregs
Proof of a duty fulfilled by the Aldrich Faithful, who patiently await the Devourer of Gods' return.
Dregs are the heaviest things within the human body, and will sink to the lowest depths imaginable.

Great Deep Soul (Sorcery)
Fires powerful darkened soul dregs.
Archdeacon McDonnell, a sorcerer himself, delighted in the cathedral's stagnating souls.
For him, they represented the glorious bedrock of this world.

Archdeacon's Great Staff
Large staff bestowed upon Archdeacons of the Cathedral of the Deep.
The Archdeacon McDonnell's trespass, the sin of channeling faith for sorcery, transformed what was once merely a symbol of ecclesiastic authority into a catalyst for sorceries.